An optical disc reproducing/recording system, such as a compact disc player (CDP), a Digital Video Disc Player (DVDP), a CD-RW (Rewritable Compact Disc), a CD-ROM (Compact Disc Read Only Memory), a CD-R (Recordable compact disc), a DVD-RAM, or a DVD-ROM inlay (damascene), generally applies laser light to a disc track to encode information in the track or read encoded information from the track. The track typically has information encoded in it so that the reflection of light from the track may be read to extract the coded information.
FIG. 1 illustrates a conventional optical disc 100 with marked sectors. As seen in FIG. 1, a sector of the optical disc 100 may have an identification unit 110 and a track unit 120. The length of the sectors is typically about several nanometers. The grooves of a track are provided in a wave or undulating pattern. The undulating pattern of a groove may be detected to generate a wobble signal at a wobble frequency. The detected wobble signal may be used as track position information when tracing a disc track and may be used as a basic signal in generating a timing clock signal. Characteristics of a wobble signal for a disc may vary with disc type.
Optical discs have conventionally been operated in two different modes. In normal speed operation, the disc may be operated in a Constant Linear Velocity (CLV) mode, where the linear velocity of the disc is maintained at a constant velocity based on where an optical pick-up assembly is reading from the disc. In such a case, the wobble signal may have a constant frequency.
Optical discs may also be operated in a high speed mode, where the disc is rotated at a constant speed. This mode is typically referred to as Constant Angular Velocity (CAV) mode. Because the disc is rotated at a constant angular velocity, the frequency of the wobble signal for tracks may vary based on the position on the disc. For example, the signal at the inner portion of the disc may be a lower frequency signal than at the outer portion of the disc because the inner portion of the disc is moving at a lower linear velocity than the outer portion of the disc. One example of the use of a wobble signal for position determination is the Address in Pre-groove (ADIP) signal that is used as data position information for DVD+R/RW. At normal speed, the ADIP signal frequency is defined as 817 KHz, while at 18× speed, the frequency is defined as 14.7 MHz.
FIG. 2 illustrates a conventional wobble signal detector 200. In the conventional wobble signal detector 200 of FIG. 2, a band pass filter (BPF) 210 is used to reduce or eliminate unnecessary noise from an input signal (RF signal) generated from a pick-up apparatus that detects a signal corresponding to a wobble signal frequency band. A comparator 220 converts an output signal of the BPF 210 into a digital signal level signal. A phase-locked loop (PLL) 230 locks to the output of the comparator to output a wobble signal.
In conventional optical disc devices, the BPF 210 may be provided by a switched capacitor filter (SCF). A SCF may provided relatively high accuracy (e.g. ±5%) and a high Q (Quality) factor. In a switched capacitor filter, a filter is provided without an inductive component by switching between capacitors. The signal that controls the switching between the capacitors may be generated using an Operational Amplifier (Op Amp) included in the BPF 210. However, the Op Amp may need to provide stable operation at 75 times the maximum frequency of the wobble signal, or up to more than 1 GHz. Furthermore, as Q for the SCF is increased, the bandwidth of the Op Amp may also need to increase. As an example, if the wobble signal to be detected is 15 MHz, the clock frequency for the SCF will typically be about 15 times the center frequency of the filter, or about 225 MHz. To assure proper operation, the bandwidth of the Op Amp may be about five times the filter frequency, or more than 1 GHz. Providing an Op Amp with such a high bandwidth may be difficult with current complementary metal oxide semiconductor (CMOS) technology because of chip size and power consumption issues. For high frequency operation, a Gm-C filter has been used as the BPF 210 in a DVD+R/RW optical disc system to detect a high frequency wobble signal. The Gm-C filter output characteristic is determined by its capacitance (C) and self transconductance (Gm). However, Gm-C filters may be sensitive to the manufacturing process, power noise, temperature variation and parasitic RC load and may provide an accuracy of only about ±30%. Thus, Gm-C filters have typically been used with a PLL with a voltage controlled oscillator (VCO) for calibration operation. However, the use of a PLL with VCO may also increase circuit size and power consumption.